Four-wheel drive vehicles can achieve a four-wheel low-speed drive mode (hereafter, referred to as ‘4L mode’) that is used when it needs a larger drive force than a two-wheel high-speed drive mode (hereafter, referred to as ‘2H mode’) or a four-wheel high-speed drive mode (hereafter, referred to as ‘4H mode’) that is used in common travel. These four-wheel drive vehicles achieve the 4L mode by generally re-decelerating the rotatory force outputted from the automatic transmission using a planetary gear set in a transfer box.
When a driver selects the 4L mode, a shift point of the automatic transmission is controlled by a shift pattern corresponding to the 4L mode.
Otherwise, at a constant vehicle speed of driving wheels, the speed of the output shaft and the engine speed become considerably higher than during normal driving due to a changed transmission gear ratio of the transfer box, such that a significant mechanical load is applied to the engine and the automatic transmission and thus durability of the power train is reduced.
Further, when the engine speed is considerably increased due to the above reason, a fuel cut-off of the engine is controlled to protect the power train. However, when a shift is performed while controlling the fuel cut-off of the engine, the shift is not smoothly performed and passengers feel a shift shock.
In order to overcome the above problems, in the related art, an individual shift pattern for a 4L mode is additionally provided. Accordingly, when a driver selects the 4L mode, the automatic transmission is controlled according to the individual shift pattern for the 4L mode, which is different from the shift pattern in a normal driving.